Alzheimer's disease is a neurodegenerative disorder characterised by progressive loss of memory and cognitive functions. It may be classified as dementia and its occurrence is such that it is the fourth most common cause of death in the industrialised nations, quite apart from the incalculable economic and social damage it causes.
In terms of anatomy and pathology, post-mortem examination of the brain tissue of an Alzheimer's patient reveals the presence of senile plaques and neurofibrillary tangles in the limbic system and the cortex. The plaques, which are located extracellularly, contain amyloid β (Aβ) peptide as their primary component, this peptide arising from amyloid precursor protein (APP) via a series of cleavages by proteolytic enzymes. Aβ peptide is produced in considerable quantities only in the brain of Alzheimer's patients and not of healthy people. The peptide, which has a relatively low molecular weight (approx. 4500 Da) and is produced in monomer form, tends to combine by means of weak interactions with other Aβ molecules, forming ever larger aggregations: oligomers, fibrils, plaques. While the monomer and oligomers are relatively soluble, fibrils and plaques are insoluble and are deposited in the brain in the form of amyloid aggregates which are then found the brain of Alzheimer's patients. These aggregates are toxic to neurons, causing them to degenerate so resulting in loss of cognitive abilities and death.
β Amyloid (Aβ) peptide, which is produced in abnormally large quantities in Alzheimer's disease, accumulates in the brain. It is assumed that a dynamic equilibrium exists: monomers⇄oligomers⇄fibrils⇄plaques. The existence of this equilibrium accounts for the increase in the quantity of insoluble aggregates in Alzheimer's brain: this is because the abnormal increase in monomer production in the disease shifts the equilibrium towards plaque formation.
β Amyloid peptide may also be found in the circulating blood, in equilibrium, through the blood-brain barrier, with that present in the brain. In confirmation of the existence of this latter equilibrium, papers published by other authors have shown that the administration of substances capable of binding Aβ peptide in the blood and removing it therefrom can also indirectly promote Aβ efflux from the central nervous system as well as amyloid plaque removal. This is known as “sink” effect. The most studied among these binding substances are antibodies injected into transgenic mice, used as a model of the disease. This is known as immunotherapy of Alzheimer's disease. The possibility of using other molecules capable of binding Aβ, such as for example the GM1 ganglioside lipid injected intravenously or the Nogo receptor injected subcutaneously, has also been tested with partial success.
If lipid molecules, or more generally molecules having amphipathic properties capable of promoting the “sink effect” when injected into the circulation, are to be used, they should be inserted into “vehicles” which are capable of enhancing their ability to interact with Aβ, while inhibiting their ability to interact with the immune system or the reticuloendothelial system which in contrast tends to eliminate these molecules from the circulation.
Liposomes are lipid vesicles made up of a double layer of amphipathic lipids enclosing an aqueous cavity. Liposome production is a simple, efficient and scalable method yielding a controlled product which is already used by the pharmaceutical industry for treating other human diseases. Moreover, using natural lipid molecules makes it possible to produce a biocompatible and biodegradable product. Liposomes have long been known as an excellent carrier system capable of incorporating both lipophilic or amphipathic molecules (in the double phospholipid layer) and hydrophilic molecules (in the internal aqueous core) and are widely used for the targeted transport of drugs and contrast agents to tissues of interest. Drug release, in vivo stability and biodistribution are determined by the size, surface charge, surface hydrophobicity and fluidity of the particle membrane. It is furthermore possible to prevent rapid uptake by the reticuloendothelial system by formulating them on the basis of natural lipid components. One feature of liposomes is the highly specific nature of the interaction between the type of lipid substrate used and the active substance to be transported. It has in fact been found that lipids with a high affinity towards one particular type of molecule are completely ineffective in binding other molecules. At the same time, even slight variations in the composition of the liposomes may greatly modify their level of activity towards the species to be bound (target). Furthermore, in the event that the target species is in equilibrium with other non-target species, an effectively working liposome must exhibit exclusive affinity towards the former species; in particular, in case of the monomer⇄oligomer⇄fibril⇄plaque equilibrium, it is important for the liposomes to have high affinity towards the “monomer” and “oligomer” component; a liposome with generalised affinity towards the various species would not significantly shift the equilibrium, while a liposome with a preferential affinity towards the higher species (fibrils, plaques) could shift the equilibrium in unwanted manner towards the formation of the species with greater pathogenic potential.
Although some examples of “sink effect” have been described experimentally, there is still no effective and reliable system available for binding beta amyloid peptide; moreover, no system is available which can readily be administered systemically; it is finally necessary to combine the above-stated effectiveness with a simple and low-cost production method which does not involve using costly synthetic and semisynthetic substances, and/or substances affecting the immune system of the patient.